February 19, 2006

Animal Diseases That Infect Humans

Fueled by the continuing, rapid spread of bird flu around the world, AP writer Andrew Bridges has an interesting piece on zoonotic diseases -- animal diseases that also infect humans. (Get more Zap*Germs detail on zoonotic diseases.)

There are 1,407 pathogens — viruses, bacteria, parasites, protozoa and fungi — that can infect humans, said Mark Woolhouse of the University of Edinburgh in Scotland. Of those, 58 percent come from animals. Scientists consider 177 of the pathogens to be "emerging" or "re-emerging." Most will never cause pandemics....Bird flu has killed at least 91 people — most of them in Asia — since 2003, according to the World Health Organization. It appears to kill about half the people it infects. However, should it mutate so it can pass from human to human, it likely will grow far less deadly, said Dr. Stanley Lemon, of the University of Texas Medical Branch at Galveston.

"It is very unlikely that it would maintain that kind of case mortality rate if it made the jump," Lemon said.

Each year, one or two new pathogens and multiple variations of existing threats infect humans for the first time. That pace appears to be unsustainable in the long run because it would imply that people run the risk of being overrun, Woolhouse told reporters.

"Humans have always been attacked by novel pathogens. This process has been going on for millennia. But it does seem to be happening very fast in these modern times," Woolhouse said.

Woolhouse argues that either many of those diseases and other afflictions will not persist in humans, or that there is something peculiar today allowing so many of them to take root in humans.

One explanation may be the recent and wide-scale changes in how people interact with the environment in a more densely populated world that is growing warmer and in which travel is faster and move extensive, Marano said. Those changes can ensure that pathogens no longer stay restricted to animals, she added. Examples from recent human history include HIV, Marburg, SARS and other viruses.

That prospect leaves open the question of what future threats await humans.

"It always surprises us. We think that avian flu will be the next emerging disease. My guess is something else might come out before that," said Alan Barrett, of the University of Texas Medical Branch at Galveston. "It's very hard to anticipate what comes next."